Disney Dining Experience. It's a card you purchase, it's good for a little more than a year (so you can use it for multiple trips), and it gives you 20% off F&B (including adult beverages -- the only discount plan that does that) for up to 10 people at one meal.
You must be either an Annual Pass holder or Florida resident to purchase the DDE card. I believe the prices are $65 for AP holders and $75 for Florida residents.
The DDE card can also be used in conjunction with DDP for alcohol and other non-DDP items like appetizers, speciality drinks, or friends/family dining with you who are not on DDP.
Starting 1/1/08, an automatic 18% gratuity will be added to all DDE checks, regardless of the size of the party.